headache sometimes updates headers of files that have not changed

[fbiville]

2 bugs for the price of one 🎉

• when there is only 1 commit, headache forced the last edition year to be the current year #wat
• headache considered pure renames (i.e. R100) as changes to take into account while they should not be

[fbiville]

Fixed via https://github.com/fbiville/headache/pull/40